We all know about the ICAO (International Civil Aviation Organisation) and the IATA (International Air Transport Association). My question is, what are the differences between these two (international) organisations? Does the ICAO implement new rules, how about the IATA? Does the IATA only monitor traffic growth and what is their relationship with the worldwide aviation safety? How is the relationship between the ICAO/IATA and also the relationship of the ICAO and IATA with other local aviation authorities, such as the UK CAA, IAA etc....

IATA is effectively a powerful lobbying body for international air carriers, while ICAO is a inter-governmental organisation which deals with regulatory aspects of national civil aviation oversight. ICAO makes recommendations and sets standards (although it has no enforcement powers) which are (generally) followed by national civil aviation authorities.

International Civil Aviation Organization -
ICAO is a branch of the United Nations... represents the different aviation authorities of UN member nations, such as US FAA, UK CAA, Germany LBA... ICAO is located in Montreal, Canada.
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International Air Transport Association -
Represents most major scheduled airlines, although some non-scheduled air carrier companies have IATA representatives... Generally deals with commercial aspect of airline operations, i.e. ticketing, interline baggage transfer, liability limits, etc...
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Happy contrails
(s) Skipper

1. The FAA also sends out recommendations. What's their role in all this?

2. Does the FAA always follow all recommendations of the ICAO? If a country
refuses to implement such recommendations (e.g. because of lack of finances), can the FAA ban them from flying to the U.S. (mainland)?

3. How does the U.S. (and who) determine(s) the category each country meets? Example, Trinidad's airport is a category 2 airport, this means their airline only has limited expansion possibilities for its operations to the U.S. (mainland). Can anyone describe the different categories? How does this work? What is the role of the ICAO in this?

ICOA in fact is part of the United Nations [you know the most visiable part of the UN is less than 5% of the total UN organization] and tries to set standards across national aviation authorities, while IATA is a airline body which is [or used to be] the largest cartel in the world [well maybe second to OPEC].

Many questions to answer...
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The FAA sends recommendations to ICAO council and member states, they might be observed or... not... all depends... The same applies to the ICAO recommendations which are suggested to the FAA... sometimes the FAA will observe the recommendations... FAA likes to keep it "their way"...
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The FAA can ban any foreign airline from flying into USA airspace, and same can be done in any foreign country to ban USA air carriers... Aerolineas became a "Category 2" airline last year because of political reasons which I am not able to disclose here... If AR orders new Boeings rather than Airbus, you can be assured that AR will be suddenly found, by the FAA, to meet "Category 1" standards.
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Category 1 permits full unrestricted operations, and applications for additional service + other destinations are supported by the FAA...
Category 2 limits operations to those currently existing, application for additional services or to other destinations are denied by the FAA. Further, there is an increase in FAA inspections of aircraft, operations and maintenance facilities...
Category 3 does not permit commercial operations into the USA...
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Now for the questions from our friend Sabena 332 -
Generally 4 letter ICAO codes are used by airline operations people, while the commercial side, inflight services, etc... use 3 letter codes... Travel agents surely always use 3 letter IATA codes...
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Honest, I am surprised to see so many people who know 4 letter codes as well as 3 letter codes, here in Argentina, most people know what EZE means, but I would be surprised if anyone outside of aviation, would know what SAEZ is... In Canada and USA, generally all it takes is put a C or a K in front of the 3 letter code to get the 4 letter code... other places, rather difficult to remember...
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To become a pilot does not require you to know 250 IATA airport codes and 175 ICAO codes... When I fly a charter to a place I normally dont fly to, I often have to ask the dispatch office what the 4 letter code is, and the flight attendants for the 3 letter code...
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Happy contrails
(s) Skipper

Dear Patrick...
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That is what most people do... 3 letter codes...
Be aware (and to confuse me even further) - that some airlines even may use a "city code" rather than a specific "airport code"... i.e. writing NYC rather than JFK, if it is the only NY City airport they fly to... PAR rather than CDG (or ORY)... LON rather than LHR (or LGW)... Let's keep it simple...
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Same thing for airplane types, I see here a lot of "travel agent" abbreviated aircraft designations... i.e. 742 for... 747-200... as a pilot, I appreciate more to see mention made of a specific type and model. i.e. 747-287...
Many of our friends here are very specific too, for airplane types, registrations, change of colors, etc... yet, so "general" describing the type...
Personally, I dont care... whatever they like... but sometimes I have to read twice...
(s) Skipper

International Civil Aviation Organization -
ICAO is a branch of the United Nations... represents the different aviation authorities of UN member nations, such as US FAA, UK CAA, Germany LBA... ICAO is located in Montreal, Canada.
xxx
International Air Transport Association -
Represents most major scheduled airlines, although some non-scheduled air carrier companies have IATA representatives... Generally deals with commercial aspect of airline operations, i.e. ticketing, interline baggage transfer, liability limits, etc...
xxx
Happy contrails
(s) Skipper

Both ICAO and IATA have their general headquarter located in Montréal, Québec, Canada